Implements feature #554.

This merge request introduces the functionality to parse List[T] options from strings using a specified separator. An example of how this can be utilized is demonstrated below:

def main(number: List[float] = typer.Option([], multiple_separator=",")):
    print(f"The sum is {sum(number)}")

#  Passing --number 1,2,3 outputs 6  
#  Works with --number 1,2 --number 3 as well. Output would be 6 again.

The implementation works by overwriting the process_value function of the click parser, doing string splitting before forwarding the result to click. This ensures full compatibility with the existing features of both Click and Typer. The feature does not allow whitespace only separators to avoid ambiguities.

Feedback is much appreciated!

Open questions:

• Currently, error handling is implemented in the constructor of typer.Option, which delays error detection until the command execution. Should we consider moving this error handling to the OptionInfo constructor to allow for earlier failure detection?
  ToDo:

• Add to doc_src
• Write tests
• Update documentation